Terahertz-wave element, terahertz-wave detecting device, terahertz time-domain spectroscopy system, and tomography apparatus

ABSTRACT

A terahertz-wave element includes a waveguide ( 2, 4, 5 ) that includes an electro-optic crystal and allows light to propagate therethrough, and a coupling member ( 7 ) that causes a terahertz wave to enter the waveguide ( 2, 4, 5 ). The propagation state of the light propagating through the waveguide ( 2, 4, 5 ) changes as the terahertz wave enters the waveguide ( 2, 4, 5 ) via the coupling member ( 7 ).

TECHNICAL FIELD

The present invention relates to terahertz-wave elements, terahertz-wavedetecting devices, terahertz time-domain spectroscopy systems, andtomography apparatuses.

BACKGROUND ART

In recent years, non-destructive sensing techniques usingelectromagnetic waves in a frequency range between 30 GHz and 30 THz(terahertz waves) have been developed.

As a terahertz-wave detecting method, a method that employs a nonlinearoptic crystal is widely used. Typical examples of nonlinear opticcrystals include LiNbOx (lithium niobate, referred to as “LN”hereinafter), LiTaOx, NbTaOx, KTP, DAST, ZnTe, and GaSe. For detecting aterahertz wave using a nonlinear crystal, a Pockels effect (which is akind of a second-order nonlinear phenomenon), which is a first-orderelectro-optic effect, is used. Specifically, when light is irradiated asprobe light onto the same location as a terahertz wave, the polarizationstate of the probe light changes in accordance with the electric fieldof the terahertz wave. The amount of change in the polarization state isdetected by a polarizing element and a light detector (see PTL 1). In anelement that uses such a nonlinear crystal, the wavelength band of theprobe light is wide so as to allow for a 0.8-μm band or even a so-calledcommunication wavelength band of 1 μm or greater, thereby advantageouslyallowing for the use of an inexpensive light source, such as a fiberlaser.

In PTL 1, the polarization of the probe light is changed by so-calledvertical operation. Since the thickness of the crystal is equivalent tothe interaction distance, the sensitivity can be increased withincreasing thickness by performing phase-matching. However, in order toachieve phase-matching with a terahertz wave in a wideband, the crystalneeds to be reduced in thickness, meaning that the sensitivity and thefrequency band are in a trade-off relationship. For improvingsensitivity by increasing the interaction distance, there has been aproposal in which the nonlinear crystal is operated horizontally (seeNPL 1). In this case, a Cerenkov phase-matching method that utilizesdispersion of the terahertz wave and the probe light within thenonlinear crystal is discussed as a phase-matching method.

There have also been proposals with regard to generating a terahertzwave by a Cerenkov phase-matching method (see PTL 2 and NPL 2).

CITATION LIST Patent Literature

-   PTL 1 Japanese Patent No. 03388319-   PTL 2 Japanese Patent Laid-Open No. 2010-204488

Non Patent Literature

-   NPL 1 2010 Annual Meeting of the Spectroscopical Society of    Japan, p. 43 (Extended Abstracts, p. 128)-   NPL 2 Opt. Express, vol. 17, pp. 6676-6681, 2009

SUMMARY OF INVENTION Technical Problem

However, in Cerenkov phase-matching discussed in NTL 1, the nonlinearcrystal used is an LN crystal having a thickness of 0.5 mm, and thepropagation state of the light input as probe light significantly variesdepending on how the crystal is coupled to the waveguide. Specifically,the input light often propagates in multiple modes and becomes anaggregate of light rays with multiple group velocities, which isproblematic in terms of response velocity. Furthermore, the time that ittakes for a terahertz wave coupled via an Si prism to reach the probelight varies in the thickness direction of the LN crystal. For example,assuming that a terahertz wave enters the LN crystal having a thicknessof 0.5 mm and a refractive index of 2.2, a time difference of about 4 psoccurs. Therefore, the frequency of a terahertz wave that can bephase-matched is limited.

With regard to detection of a terahertz wave using a high-sensitivitynonlinear optic crystal of a horizontal operation type, the presentinvention allows for a wider detectable terahertz-wave frequency band byexpanding the band that can be phase-matched.

Solution to Problem

A terahertz-wave element according to an aspect of the present inventionincludes a waveguide that includes an electro-optic crystal and allowslight to propagate therethrough; and a coupling member that causes aterahertz wave to enter the waveguide. A propagation state of the lightpropagating through the waveguide changes as the terahertz wave entersthe waveguide via the coupling member.

Advantageous Effects of Invention

A high-sensitivity, wideband terahertz-wave detecting element can beprovided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are structural diagrams of a terahertz-wave elementaccording to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a tomography apparatus according tothe first embodiment of the present invention.

FIGS. 3A and 3B are diagrams for explaining Cerenkov phase-matching.

FIG. 4 illustrates an example of a terahertz waveform according to thefirst embodiment of the present invention.

FIGS. 5A and 5B are structural diagrams of a terahertz-wave elementaccording to a second embodiment of the present invention.

FIG. 6 is a structural diagram of a terahertz-wave element according toa third embodiment of the present invention.

FIG. 7 is a structural diagram of a terahertz-wave element according toa fourth embodiment of the present invention.

FIGS. 8A and 8B are structural diagrams of a terahertz-wave elementaccording to a fifth embodiment of the present invention.

FIG. 9 is a configuration diagram of a tomography apparatus according tothe fifth embodiment of the present invention.

FIGS. 10A and 10B are structural diagrams of a terahertz-wave elementaccording to a sixth embodiment of the present invention.

FIG. 11 is a structural diagram of another terahertz-wave elementaccording to the sixth embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS First Embodiment

A terahertz-wave element composed of an LN crystal according to a firstembodiment of the present invention will now be described with referenceto FIGS. 1A and 1B. FIG. 1A is a perspective view, and FIG. 1B is across-sectional view taken along line IB-IB in a waveguide section.

An LN substrate 1 is composed of Y-cut lithium niobate, and the LNcrystal has an X axis corresponding to a propagating direction of laserlight and a Z axis corresponding to a direction orthogonal to thepropagating direction (see coordinate axes shown in FIG. 1A). With sucha configuration, a change in refractive index effectively occurs due toa first-order electro-optic effect (Pockels effect) by a terahertz wave,denoted by reference numeral 12 in FIG. 1A, entering as an S-polarizedwave (i.e., a linearly-polarized wave parallel to the Z axis of the LNcrystal in this embodiment). On the LN substrate 1, an adhesive layer 2,a waveguide layer 4 formed of an MgO-doped LN crystal layer, and alow-refractive-index buffer layer 5 form a waveguide that allows inputlaser light to propagate therethrough by total internal reflection.Specifically, the refractive indices of the adhesive layer 2 and thelow-refractive-index buffer layer 5 are set to be lower than that of thewaveguide layer 4. The waveguide layer 4 is a core layer serving as acore for the laser light, whereas the adhesive layer 2 and the bufferlayer 5 are cladding layers serving as cladding for the laser light. Theadhesive layer 2 is required if the waveguide is to be fabricated bybonding the components together, but is not necessarily required if adoped layer is to be formed by diffusion or the like. Even in this case,the function of a waveguide is still achieved since the MgO-doped LNlayer has a refractive index that is higher than that of the LNsubstrate 1. The waveguide can be formed by varying the refractiveindices between the waveguide layer 4 and a surrounding region 3 byincreasing the refractive index of the waveguide layer 4 by Tidiffusion, or by forming the waveguide layer 4 into a ridge pattern byetching and embedding resin or the like into the surrounding region 3.Alternatively, the surrounding region 3 may be kept in a state of voidwithout embedding anything therein. Although the waveguide structure isalso formed in the lateral direction, like the waveguide layer 4, so asto increase the light confinement properties, a slab waveguide in whichthe region of the waveguide layer 4 extends uniformly in the lateraldirection so as to not to have a confinement region, like thesurrounding region 3, is also permissible. Alternatively, multiplewaveguide layers 4 may be arranged in parallel to each other in thelateral direction so as to increase a terahertz-wave light-receivingregion while controlling the light waveguide mode. On thelow-refractive-index buffer layer 5, an optical coupling member 7, suchas a prism, a diffraction grating, or a photonic crystal, that couples aterahertz wave to be detected to the waveguide from the outside isprovided at least above the waveguide. The thickness of the buffer layer5 is preferably large enough to function as a cladding layer when thelaser light propagates through the waveguide layer 4, but small enoughthat the effect of multiple reflection and loss is negligible when theterahertz wave enters the optical coupling member 7. Regarding theformer, in the waveguide in which the waveguide layer 4 as ahigh-refractive-index layer serves as a core whereas thelow-refractive-index layers 2 and 5 serve as cladding, theaforementioned thickness is preferably greater than or equal to athickness with which the light intensity at an interface between thelow-refractive-index buffer layer 5 and the optical coupling member 7 islower than or equal to 1/e² of the light intensity of a core region. Itshould be noted that e is the base of natural logarithm. Regarding thelatter, the thickness is preferably smaller than or equal to about 1/10of an equivalent wavelength λ_(eq) (THz), in the low-refractive-indexbuffer layer 5, of an input terahertz wave at the maximum frequency.This is because, in a structural body with a size corresponding to 1/10of a wavelength, the effects of reflection, dispersion, refraction, andthe like on an electromagnetic wave with that wavelength are generallyconsidered to be negligible. It should be noted, however, thatterahertz-wave detection using the terahertz-wave element according tothe present invention is possible even outside the aforementionedpreferred thickness range.

Cerenkov phase-matching will now be described with reference to FIG. 2.This can be easily appreciated on the basis of the concept ofelectro-optic Cerenkov radiation that generates a terahertz wave from anonlinear optic crystal. In FIG. 2, when the propagation velocity oflaser light 100 serving as an excitation source is higher than thepropagation velocity of a generated terahertz wave, a terahertz wave 101is released in the form of a cone, like a shock wave. In the case of anormal electro-optic crystal bulk body, a radiation angle θc (i.e., anangle θc between the light and the terahertz wave) is determined fromthe following equation (1), which is the ratio of refractive indices ofthe light and the terahertz wave within the medium (i.e., nonlinearoptic crystal).

$\begin{matrix}{{\cos\;\theta_{c}} = {\frac{v_{THz}}{v_{g}} = \frac{n_{g}}{n_{THz}}}} & (1)\end{matrix}$

In this case, vg and ng respectively denote the group velocity and thegroup refractive index of excitation light relative to the nonlinearoptic crystal, and v_(THz) and n_(THz) respectively denote the phasevelocity and the refractive index of a terahertz wave relative to thenonlinear optic crystal. For example, there has been a report in NPL 2(PTL 2) with regard to generating a wideband monochromatic terahertzwave by a difference frequency generation method based on Cerenkovphase-matching by using a slab waveguide having a thickness that issufficiently smaller than the wavelength of a generated terahertz wave.In this case, ng and n_(THz) are effective refractive indices of thelight and the terahertz wave. For example, ng is the group refractiveindex of the waveguide relative to the light. If a prism for opticalcoupling exists in the vicinity of the waveguide as in NPL 2 (PTL 2), itis also necessary to consider radiation in view of the refractive indexof the prism. Therefore, n_(THz) is an effective refractive index of theterahertz wave determined from the waveguide and the coupling member(prism). If the waveguide is thin, θc can be adjusted by selecting anappropriate material for the prism.

Regarding a reverse process of this generation process, when thewavefront of the terahertz wave is returning, a relationship that is thesame as that in the aforementioned equation (1) should be satisfied inorder to cause a point of a front 102 of the laser light 100 toconstantly interact with the terahertz wave while similarly returning tothe laser source. This is called Cerenkov phase-matching that is usedwhen detecting a terahertz wave. In this case, it is important that, forexample, an interaction occurs between the terahertz wave and the laserlight at the point 102. Therefore, when the propagating area of thelaser light reaches a thickness substantially equivalent to thewavelength of the terahertz wave (e.g., a width between 100 and 100′),the interaction point blurs at the wavefront of the interactingterahertz wave due to a time difference between points above and below(102 and 102′) the aforementioned propagating area. When such blurringoccurs, the terahertz-wave element becomes incapable of responding to ahigh-speed change, that is, a high frequency, of the terahertz wave. Aquantitative description of the thickness will be provided later.

Next, a detection mechanism of a detecting unit will be described. Whenlinearly-polarized laser light tilted at, for example, 45° relative tothe Z axis enters the waveguide layer 4 and propagates therethroughalong the X axis, as shown in FIG. 1, the polarization state changes dueto birefringence of the LN crystal even in a state where a terahertzwave is not input (natural birefringence). The polarization state of thepropagated light output from a surface different from the input surfacecan be checked by performing balance reception using a Wollaston prism 9and two photodiodes 10 and 11. Regarding the output laser light in thiscase, a phase difference occurs between a Z-axis component and a Y-axiscomponent of an electric field due to birefringence of the electro-opticcrystal, causing the light to propagate as an elliptically polarizedwave. The phase difference occurring due to such natural birefringencevaries depending on the type of crystal (LN is 3 m crystal), the inputpolarization direction, and the waveguide length, and a configurationfor zero phase difference is also possible. A polarization variation ina state where there are no terahertz signals may be adjusted using aknown compensating plate (not shown) so as to cancel offsets. Since adetailed description is provided in PTL 1, the description will beomitted here.

With regard to the interaction occurring when a terahertz wave is inputin this embodiment, the interaction is made to occur by utilizing achange in the polarization state of the propagating light owing to achange in the refractive index of the waveguide on the Z axis caused bya first-order electro-optic effect imparted on the electro-optic crystalby a terahertz electromagnetic field. Specifically, the phase differencebetween the Z-axis component and the Y-axis component of the electricfield of the laser light changes due to induced birefringence, causingthe ellipticity of the elliptically polarized wave and the direction ofthe main axis to change.

By detecting the change in the propagation state of the laser light onthe basis of differential amplification by using an external polarizingelement (such as the Wollaston prism 9) and light detectors (such as thephotodiodes 10 and 11), the electric-field amplitude of the terahertzwave can be detected. The differential amplification is not mandatory,and the intensity may be detected with only a single light detectorusing the Wollaston prism 9 as a polarizer.

Furthermore, although a method for detecting a change in thepolarization state of propagating light has been described above, thereis also a method for detecting a change imparted on the propagatinglight by an interaction between a terahertz wave generated by thepropagating light and a terahertz wave input from the outside as achange in the oscillation frequency or the light intensity of thepropagating light. In that case, light detectors alone are sufficientand the polarizing element is not necessary, and the polarization planeof input light may be parallel to the Z axis.

Although the detecting unit described above is necessary when convertinga terahertz-wave signal to an electric signal, the detecting unit is notnecessary if the laser light itself is modulated and the modulated lightis used at a later stage. This applies to when, for example, theterahertz-wave element according to the present invention is used as alight modulator that uses a terahertz wave.

FIGS. 3A and 3B illustrate an example of a tomographic imaging apparatus(tomography apparatus) based on a terahertz time-domain spectroscopysystem (THz-TDS) that uses the terahertz-wave element according to thisembodiment as a terahertz-wave detecting element.

A femtosecond laser 20 including optical fibers is used as an excitationlight source, and an output therefrom is extracted from a fiber 22 and afiber 23 via a splitter 21. Although the femtosecond laser 20 usednormally has a center wavelength of 1.55 μm, a pulse width of 20 fs, anda cyclic frequency of 50 MHz, the wavelength may alternatively be in a1.06-μm band, and the pulse width and the cyclic frequency are notlimited to the aforementioned values.

Furthermore, the fibers 22 and 23 at the output stage may each include ahigh nonlinear fiber for high-order soliton compression at the finalstage or a dispersive fiber that performs prechirping for compensatingfor dispersion caused by an optical element or the like extending to aterahertz generator and a terahertz detector. These fibers arepreferably polarization-maintaining fibers.

The terahertz-wave detection side is coupled to the waveguide of aterahertz-wave element 24 according to this embodiment described above.In this case, the terahertz-wave detection side may be spatially coupledto the light from the femtosecond laser 20 using a lens via an opticaldelay unit 29. Alternatively, a delay unit (not shown) using a fiberstretcher for achieving an all fiber configuration may be used, or anoptical delay unit may be disposed at the terahertz-wave generationside. In that case, the detection side may be constituted by integratinga Selfoc lens with the fiber end or may be of a pigtail type formed byprocessing the aforementioned end so that the numerical aperture thereofis smaller than or equal to the numerical aperture of the waveguide ofthe Cerenkov phase-matching element. In this case, the ends may each beprovided with a nonreflective coating so as to reduce Fresnel loss andundesired interference noise. Alternatively, by designing the fiber 23and the waveguide so that they have similar numerical apertures andsimilar mode field diameters, direct coupling (butt-coupling) byabutment is also permissible. In this case, an adhesive is appropriatelyselected so that an adverse effect caused by reflection can be reduced.

If the fiber 22 or the femtosecond laser 20 at the preceding stageincludes a non-polarization-maintaining fiber component, it ispreferable to stabilize the polarization of input light entering theterahertz-wave element 24 according to the present invention by using aninline-type polarization controller.

However, the excitation light source is not limited to a fiber laser,and in that case, the countermeasure for stabilizing the polarization isreduced.

A terahertz wave is generated by, for example, irradiating light outputfrom the optical fiber 22 onto a photoconductor 28, is made into acollimated beam by a parabolic mirror 26 a, and is split by a beamsplitter 25. One of the split beams is irradiated onto a sample 30 via aparabolic mirror 26 b. The terahertz wave reflected from the sample 30is collected by a parabolic mirror 26 c and reaches the terahertz-waveelement 24 and a detector 27. The photoconductor 28 used is normally adipole antenna formed using low-temperature growth InGaAs if the centerwavelength of the light source corresponds to 1.55 μm. As mentionedabove, the detector 27 includes, for example, the Wollaston prism 9 andthe two photodiodes 10 and 11.

The apparatus is configured to modulate voltage applied to thephotoconductor 28 by a power source unit 31 and synchronously detect anoutput from the detector 27, which detects light whose propagating statehas been changed by the terahertz-wave element 24, by using a signalacquiring unit 32 via an amplifier 34. A data processing and output unit33 is configured to acquire a terahertz signal waveform while using apersonal computer or the like to move the optical delay unit 29. FIG. 4illustrates an example of a terahertz pulse waveform acquired only fromsurface reflection when the sample 30 is a mirror.

In this system, the reflected wave from the sample 30 to be measured andthe irradiated terahertz wave are coaxial with each other, and the powerof the terahertz wave is reduced by half. Therefore, the irradiatedterahertz wave and the reflected wave may be made non-coaxial with eachother by increasing the number of mirrors, as in FIG. 3B, so as toincrease the power of the terahertz wave, although the incident angle onthe sample 30 in this case becomes unequal to 90°.

If there is a discontinuous section in the material inside the sample30, a signal to be acquired would have a reflective echo pulse occurringat a time position corresponding to the discontinuous section. Thus, atomographic image can be obtained by one-dimensionally scanning thesample 30, or a three-dimensional image can be obtained bytwo-dimensionally scanning the sample 30.

With the tomography apparatus according to this embodiment, the internalpenetration depth and the depth resolution can be improved. Furthermore,since an excitation laser using fibers can be used as an irradiatingunit, the apparatus can be reduced in size and cost.

Although light is input from an end opposite to the generation side inthis embodiment, light may alternatively be input from the same side asthe generation side. In that case, the signal strength becomes smallersince the matching length is reduced.

Although the excitation light source used is of an ultra-short pulsetype, the above embodiment can also be applied to a case where asingle-wavelength terahertz wave based on differential frequencygeneration is to be detected with a continuous wave or ananosecond-order pulse by using two lasers having different wavelengths.

Although an LN crystal is used as the material for the crystal, otheralternative examples of electro-optic crystals include LiTaOx, NbTaOx,KTP, DAST, ZnTe, and GaSe, as mentioned in the background art.

Example 1

In the element structure shown in FIGS. 1A and 1B in this example, theoptical adhesive layer 2 having a refractive index n of about 1.5 isformed with a thickness of 3 μm, the waveguide layer 4 composed of anMgO-doped LN crystal is formed with a thickness of 3.8 μm and a width of5 μm, and the low-refractive-index buffer layer 5 composed of the sameoptical adhesive as that used for the optical adhesive layer 2 is formedwith a thickness of 3 μm. A high-resistivity Si prism is used as theoptical coupling member 7. Specifically, in order to satisfy Cerenkovphase-matching, a prism with an angle θ of 41° is attached so that aterahertz wave can orthogonally enter the prism surface at an angleθclad of 49°. Although the surfaces that are not the terahertz-waveinput surface appears as if they are inclined, the angle of thesesurfaces is arbitrary, such as a right angle. Although the length of thewaveguide is set to, for example, 10 mm, the length is not limited tothis value.

The thickness of the waveguide layer 4 is determined as follows.Specifically, a maximum frequency fmax is determined from a Fourierfrequency band to be detected as a terahertz pulse. Then, the thicknessof the waveguide layer 4 is set such that the thickness is smaller thanor equal to half the length of an equivalent wavelength within thecrystal, corresponding to the maximum frequency fmax, and that a singlemode condition corresponding to a good coupling efficiency and a lowpropagation loss of input ultra-short pulse laser light is satisfied. Inorder to handle up to, for example, 7.5 THz in this example, thewavelength in a free space is about 40 μm, and if the refractive indexof the terahertz wave in the LN waveguide layer is 5.2, the thickness ofthe waveguide layer is preferably λ_(eq)(THz)/2(=40/5.2/2)=3.8 μl. Onthe other hand, in view of coupling efficiency and propagation loss,waveguide simulation results show that the optical waveguide in thisexample preferably has a thickness of about 5 μm if the centerwavelength of the input laser light is 1.55 μm. However, the lowercondition, that is, the thickness of 3.8 μm for the waveguide, ispreferentially selected so as to ensure a terahertz wave band. In thiscase, fmax=7.5 THz in this example corresponds to a frequency of LOphonon absorption of the LN crystal and is set in view of the fact thatthe terahertz wave is significantly absorbed and not released near thatfrequency. There are cases where detection of, for example, 10 THz orhigher, which is a frequency that is higher than the LO phononabsorption band, is possible depending on the pulse width of the inputlaser light. In that case, the thickness of the optical waveguide isreduced accordingly. Furthermore, if the center wavelength of the inputlaser light is 1 μm, an optimal thickness is about 3.6 μm based onsimulation. In this case, this thickness is selected. It is thusimportant to determine the thickness of the waveguide layer 4 in view ofthe differences in the required terahertz band or the condition of agood coupling efficiency and a low propagation loss of input laserlight, and it is preferable to select the lower one of these twoconditions as an optimal mode.

On the other hand, the low-refractive-index buffer layer 5 with athickness of 2 μm is formed using the same optical adhesive as that usedfor the optical adhesive layer 2. Similarly, in order to handle up to7.5 THz, assuming that the equivalent wavelength is equal to a valuedivided by the refractive index 1.5 of the low-refractive-index bufferlayer 5, the thickness is set to 2 μm, which is smaller than or equal toλ_(eq)/10 (=40/1.5/10)=2.7 μM, as mentioned in the first embodiment.

Second Embodiment

A second embodiment of the present invention will now be described withreference to FIGS. 5A and 5B. This embodiment uses a sandwich-type slabwaveguide as a waveguide layer 42 through which laser light propagates,and does not have an LN substrate that holds the waveguide layer 42. Thelength of the waveguide is set to, for example, 5 mm. FIGS. 5A and 5Bdiffer from FIGS. 1A and 1B in that the prism side is shown as the lowerside.

The sandwich-type slab waveguide can be achieved by preparing a bondingwafer 45, as shown in FIG. 5B. The bonding wafer 45 is formed byadhering an MgO-doped LN crystal substrate 42′, which is to become awaveguide, onto a high-resistivity Si substrate 40′, which is a materialto become a prism 40, by using an adhesive 41′ (the same as that in thefirst embodiment), which is to become a low-refractive-index bufferlayer. The LN crystal substrate 42′ is ground until reaching thethickness of the waveguide. After the grinding, a low-refractive-indexlayer 43 made of resin or an oxide film, such as SiO₂, for protection ispreferably formed on the waveguide layer 42 (FIG. 5A). Even if thislow-refractive-index layer 43 is not to be provided, light can still beconfined in the waveguide layer since the refractive index of air islow.

An Si prism may be given an inclined section by grinding or chemicaletching. For example, in the case of a (100) Si substrate, known wetetching (such as KOH) may be performed so as to form a (111) surfacewith a 55° inclination angle. Although this surface deviates by 14° froman ideal surface having a 41° inclination angle, an increase inreflection loss (Fresnel loss) at the surface is minimal. It is needlessto say that an inclined substrate may be used for achieving a 41°surface.

The input light may be in the shape of an oval, as denoted by referencenumeral 44. In that case, a rod lens or a cylindrical lens may be usedas a lens for coupling the light from the laser light source so that thelight is throttled in the vertical direction of the layered structure ofthe waveguide.

Although a light detecting unit in the later stage is omitted, theterahertz-wave detecting method is the same as that in the firstembodiment.

The use of the slab waveguide in this embodiment advantageouslyfacilitates the coupling of probe light and allows for a wideinteraction area even when the terahertz wave is insufficientlycollected.

Third Embodiment

In a third embodiment of the present invention, the detectors are alsointegrated on the same substrate, as shown in FIG. 6. A substrate 50 isprovided with a terahertz-wave element 51 based on Cerenkovphase-matching, which is similar to that in the first embodiment. Anoutput end of the terahertz-wave element 51 is provided with awaveguide-type polarizing beam splitter 52 that splits input light intotwo polarized components and guides the two polarized components to twointegrated optical waveguides 53 and 54. Moreover, two light detectors55 and 56 are integrated on the substrate 50 so as to detect lightoutput from the waveguides 53 and 54. The outputs from the two lightdetectors 55 and 56 are used for detecting a terahertz-wave signal bybalance reception, as in the first embodiment.

The waveguide-type polarizing beam splitter 52 can be achieved byforming a dielectric multilayer film at a Y-branch section of thewaveguides 53 and 54. Furthermore, regarding the waveguides 53 and 54,the substrate 50 may be composed of Si, and Si waveguides may befabricated by forming rectangular patterns on the Si substrate 50. If anexcitation laser with a 1-μm band or lower is used, since the light isabsorbed by Si, SiO₂ waveguides may be used. Regarding the lightdetectors 55 and 56, MSM detectors based on InGaAs may be integrated onthe substrate 50.

In the case where the substrate 50 is composed of Si, as describedabove, the Si substrate may be etched as in the second embodiment so asto allow a terahertz wave to enter from the rear side (not shown) in theplan view of FIG. 6.

In this embodiment, the number of spatial coupling systems is reduced sothat the element itself is made compact and stable, therebyadvantageously reducing a loss of light when being guided to the lightdetectors 55 and 56. By using the element according to this embodimentas the terahertz-wave detecting element in the tomography apparatusdescribed in the first embodiment, imaging performance can be enhanced.

Fourth Embodiment

As shown in FIG. 7 in plan view, a fourth embodiment according to thepresent invention provides an integrated element that includes aMach-Zehnder interferometer 64 having a Y-branch section 67 and acoupler 68, and that causes an output from the Mach-Zehnderinterferometer 64 to propagate through a waveguide 63 so as to acquire asignal using a light detector 66. The material used here can be anonlinear optic crystal mainly composed of LN as in the aboveembodiments. The Y-branch section 67 branches a waveguide 61 into adetection waveguide (detection optical path) through which light to bedetected propagates and a reference waveguide (reference optical path)through which reference light propagates. The waveguide in aterahertz-wave element 62 according to the present invention is includedin the detection waveguide.

In this embodiment, a terahertz wave is detected by utilizing a changein the phase state of light instead of utilizing a polarizationvariation of light propagating through the waveguide.

Therefore, although the crystal-axis direction of the waveguide in theterahertz-wave element (Cerenkov phase-matching section) 62 is the sameas that in the first embodiment, the polarization direction of inputlaser light 69 is set parallel to the Z axis. In that case,birefringence does not occur at the MgO-doped LN crystal layer, and apolarization variation does not occur even when the waveguides 61 and 63are formed of this crystal.

When a terahertz wave enters as an S-polarized wave, the propagationvelocity of propagating light changes since the refractive index changesdue to the Pockels effect. Due to the Mach-Zehnder interferometerconfiguration, a phase difference occurs between the detection opticalpath and the reference optical path at the coupler 68 when thepropagation velocity in one of the waveguides changes, causing a changein the light intensity due to interference.

Because this phase difference changes in accordance with the intensityof an input terahertz wave, a terahertz signal can be received by thelight detector 66.

Since a polarization controller is not used in this embodiment, thestructure is simplified. By using the element according to thisembodiment as the terahertz-wave detecting element in the tomographyapparatus described in the first embodiment, a compact and stable systemcan be achieved.

Fifth Embodiment

A fifth embodiment according to the present invention utilizesterahertz-wave generation using electro-optic Cerenkov radiationdescribed in the first embodiment and provides a structure integratedwith the terahertz-wave detecting element based on Cerenkovphase-matching according to the present invention.

An example of the structure is illustrated in a cross-sectional view inFIG. 8A and a plan view in FIG. 8B. FIG. 8A is a cross-sectional view ofa waveguide section through which laser light propagates. As shown inFIG. 8B in plan view, two waveguides 84 a and 84 b each formed of anMgO-doped LN crystal similar to that used in the first embodiment areprovided parallel to each other in a single element. Reference numerals81, 82, and 85 denote an LN substrate and upper and lowerlow-refractive-index adhesive layers, respectively, as in the firstembodiment. Reference numeral 87 denotes an Si prism. As shown in FIGS.8A and 8B, one of the waveguides (in this case, the waveguide 84 a)corresponds to a known Cerenkov terahertz-wave generating element fromwhich a terahertz wave is released to the space at an angle θclad of 49°due to laser light propagation. The other waveguide corresponds to aterahertz-wave detecting element according to the present invention andis configured to couple a terahertz wave to the waveguide 84 b at anangle θclad of 49° via an Si prism 87 b. Therefore, the waveguide 84 aand the waveguide 84 b are respectively provided with Si prisms (i.e., afirst coupling member 87 a and a second coupling member 87 b) that aredifferent from each other (specifically, inclined in differentdirections from each other). The inclination angles may be the same asin the first embodiment, and in that case, crosstalk between a generatedterahertz wave and a detected terahertz wave is small. Probe light isinput as second laser light to the waveguide 84 b at the detection side,and input polarized light is tilted at 45° as in the first embodiment. Apolarization splitter 83, such as a Wollaston prism, and two lightdetectors 86 and 88 are disposed at the output end so that thepropagation state of the laser light changed due to the terahertz-wavesignal can be acquired. Although the two waveguides 84 a and 84 b eachhave a width of 5 μm and are spaced apart from each other by about 1 mm,the waveguides 84 a and 84 b are not limited to this configuration. Thewaveguides 84 a and 84 b may alternatively be a slab waveguide having ageneration section and a detection section that are separated from eachother at a laser-light input position.

An example of a terahertz time-domain spectroscopy system using such anintegrated Cerenkov phase-matching element is illustrated in FIG. 9. Anoutput from a fiber laser 70 is split into two components via a coupler71, and one of the split components is guided as pump light (first laserlight in FIGS. 8A and 8B) to an integrated Cerenkov phase-matchingelement 73 according to this embodiment via an optical fiber 79. Theother split laser-light component is coupled to an optical fiber 78 bfrom an optical fiber 78 a via a delay optical unit 72 and a lightchopper 91 and is guided as probe light (second laser light in FIGS. 8Aand 8B) for the integrated Cerenkov phase-matching element 73. Agenerated terahertz wave is collected by a parabolic mirror 76 a and isirradiated onto a sample 90. The terahertz wave reflected at the sample90 is collected by a parabolic mirror 76 b and is detected by theelement 73 and a detector 74. The detector 74 includes the polarizationsplitter 83 and the two light detectors 86 and 88 shown in FIGS. 8A and8B.

A tomographic-image acquiring method and a spectroscopy-informationacquiring method using this system can be achieved by using knownmethods.

The fifth embodiment can provide an element in which a generator and adetector are integrated into a single unit, thereby providing a compactterahertz time-domain spectroscopy system.

Sixth Embodiment

A sixth embodiment of the present invention relates to a configurationin which a coupling member is given a curved surface to enhancecollectability of a terahertz wave. FIGS. 10A and 10B illustrate astructural body 92, serving as a coupling member, formed by cutting twofaces of a hyper-hemispherical lens. In FIGS. 10A and 10B, componentssimilar to those in FIGS. 1A and 1B are given the same referencenumerals. In this case, since a terahertz wave is collected at a focalpoint of the lens, the terahertz wave is input from a directiondifferent from that in the first embodiment. Specifically, the laserlight and the terahertz wave are input from opposite directions. If anultra-short pulse with several tens of fs or shorter is used as thelaser light, since the pulse width expands within the waveguide due towavelength dispersion, there are cases where a wideband can be achievedbetter by causing an interaction to occur near the input end.

Therefore, the terahertz wave is input at an angle θclad (49° if an Simember is used), as shown in FIG. 10B, and the focal point at theleading end of the arrow is set at, for example, a position that is 500μm inward from an end of the waveguide.

By giving the waveguide a circular-arc-like curve in a cross sectiontaken in a direction orthogonal to the propagating direction of thelaser light, a terahertz wave from the lateral direction of thewaveguide can also be collected, thereby increasing the utilizationefficiency of the terahertz wave, as compared with when a triangularprism is used as in the first embodiment. Alternatively, the laser lightand the terahertz wave may be input from the same direction (as in thefirst embodiment) by inverting the orientation of the lens.

As another structure in which the waveguide is given a circular-arc-likecurve in a cross section taken in a direction orthogonal to thepropagating direction of the laser light, a structural body 93 formed bycutting one face of a cone is also permissible, as shown in FIG. 11. Inthis case, a terahertz wave can be collected linearly without a focalpoint in the direction of the waveguide while also collecting aterahertz wave from the lateral direction of the waveguide. Therefore,the laser light and the terahertz wave can be input from the samedirection as in the first embodiment, thereby allowing for an increasedinteraction distance. Consequently, the S/N ratio can be furtherimproved, as compared with the case where a cut hyper-hemispherical lensis used as described above. The structural body 92 or the structuralbody 93 may be selected on the basis of the relationship between the S/Nratio and the band of a signal to be detected.

Although the waveguide and the coupling member are illustrated in FIGS.10A to 11 as if their lengths match, the lengths do not necessarily needto match.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-006123 filed Jan. 14, 2011 and No. 2011-230004 filed Oct. 19, 2011,which are hereby incorporated by reference herein in their entirety.

REFERENCE SIGNS LIST

-   -   1 substrate    -   2 adhesive layer    -   3 waveguide surrounding region    -   4 waveguide layer    -   5 low-refractive-index buffer layer    -   7 optical coupling member

The invention claimed is:
 1. A terahertz-wave element comprising: awaveguide that includes an electro-optic crystal and allows light topropagate therethrough; and a coupling member that causes a terahertzwave to enter the waveguide, wherein a propagation state of the lightpropagating through the waveguide changes as the terahertz wave entersthe waveguide via the coupling member, wherein the waveguide includes acore layer and a cladding layer, wherein the cladding layer isinterposed between the coupling member and the core layer, and whereina<d<λ_(eq)/10 is satisfied where d denotes a thickness of the claddinglayer, a denotes a thickness corresponding to 1/e² of a light intensityof the light in the core layer, e being the base of natural logarithmand λ_(eq) denotes an equivalent wavelength in the cladding layer withrespect to a wavelength corresponding to a maximum frequency of theterahertz wave.
 2. The terahertz-wave element according to claim 1,wherein the waveguide includes a core layer and a cladding layer, andwherein a thickness of the core layer is smaller than or equal to half alength of an equivalent wavelength in the core layer, corresponding to amaximum frequency of the terahertz wave.
 3. The terahertz-wave elementaccording to claim 1, wherein an angle θ_(c) formed between the lightand the terahertz wave satisfies the following equation:${\cos\;\theta_{c}} = \frac{n_{g}}{n_{THz}}$ where ng denotes aneffective group refractive index of the light, and n_(THz) denotes aneffective refractive index of the terahertz wave.
 4. A terahertz-wavedetecting device comprising: the terahertz-wave element according toclaim 1; and a detecting unit configured to detect the light propagatingthrough the waveguide of the terahertz-wave element.
 5. Theterahertz-wave detecting device according to claim 4, wherein thedetecting unit detects a polarization state of the light.
 6. Theterahertz-wave detecting device according to claim 5, wherein theelectro-optic crystal includes Y-cut lithium niobate, wherein if apropagating direction of the light in the waveguide is defined as an Xaxis and a direction orthogonal to the propagating direction is definedas a Z axis, the light input to the waveguide is polarized lightincluding a Y-axis component and a Z-axis component of an electricfield, and wherein the detecting unit includes a polarization splitterthat splits the light propagating through the waveguide intolinearly-polarized light components polarized in different directions,and a light detector that detects the linearly-polarized lightcomponents.
 7. The terahertz-wave detecting device according to claim 6,wherein the polarization splitter, the light detector, and the waveguideare formed on a single substrate, and wherein the polarization splitterand the light detector are coupled to each other by the waveguide. 8.The terahertz-wave detecting device according to claim 5, wherein thedetecting unit detects a phase state of the light.
 9. A terahertztime-domain spectroscopy system comprising the terahertz-wave detectingdevice according to claim
 4. 10. A tomography apparatus comprising theterahertz time-domain spectroscopy system according to claim
 9. 11. Theterahertz-wave element according to claim 1, wherein, in a cross sectiontaken in a direction orthogonal to a propagating direction of the lightin the waveguide, the coupling member has a curve in a portion thereofother than a surface thereof that is in contact with the electro-opticcrystal.
 12. A terahertz-wave element comprising: a first waveguideincluding an electro-optic crystal; a second waveguide including anelectro-optic crystal; a first coupling member that extracts a terahertzwave, which is generated from the electro-optic crystal due to lightpropagating through the first waveguide, to a space; and a secondcoupling member that causes the terahertz wave to enter the secondwaveguide from the space.
 13. A terahertz-wave detecting devicecomprising: i) a terahertz-wave element including: a waveguide thatincludes an electro-optic crystal and allows light to propagatetherethrough; and a coupling member that causes a terahertz wave toenter the waveguide, wherein the waveguide includes a core layer, andwherein a thickness of the core layer is smaller than or equal to half alength of an equivalent wavelength in the core layer, corresponding to amaximum frequency of the terahertz wave; and ii) a detecting unitconfigured to detect the light propagating through the waveguide of theterahertz-wave element.
 14. The terahertz-wave detecting deviceaccording to claim 13, wherein the detecting unit detects a polarizationof the light.
 15. The terahertz-wave detecting device according to claim13, wherein the detecting unit detects a phase state of the light.
 16. Aterahertz-wave element comprising: a waveguide that includes anelectro-optic crystal and allows light to propagate therethrough; and acoupling member that causes a terahertz wave to enter the waveguide,wherein a propagation state of the light propagating through thewaveguide changes as the terahertz wave enters the waveguide via thecoupling member, wherein the waveguide includes a core layer, wherein athickness of the core layer is smaller than or equal to half a length ofan equivalent wavelength in the core layer, corresponding to a maximumfrequency of the terahertz wave.